In a conventional eddy current flaw detecting apparatus and method, types of flaws, such as an external flaw, an internal flaw and a through hole flaw, in an object being examined are judged by a phase analysis method of flaw signals from a bridge circuit wherein the analyzed data are compared with those caused by flaws with a standard flaw testing piece obtained in advance while varying the phase angle .PHI. of a flaw signal processing unit of an eddy current flaw detecting apparatus, as shown in, for example, Eddy Current Flaw Detecting Test A (edited by Nihon Hihakai Kensa Kyokai (Japan Non-Destructive Inspection Society), 1977). In order to determine size of flaws by using the same flaw detecting apparatus, the relationship between the flaw signal output (V) and the size of a flaw, in depth thereof or in diameter of through hole, for each type of flaw in the standard flaw testing piece is obtained in advance, and the size of a flaw of an object being examined is determined comparing the obtained relationship with that in the standard flaw testing piece.
Although the position, type and size of a flaw of the workpieces can be determined by the above-described conventional apparatus and method, it was impossible to determined the direction of the flaw, in other words, whether the flaw is an axially orienting flaw or a circumferentially orienting flaw.
For example, when the object being examined is a fuel clad pipe for light-water type power reactor, if it is assumed that a rise in the inner pressure thereof would break the clad pipe, an axial flaw exerts a deleterious effect rather than a circumferential flaw. However, depending upon objects being examined, the circumferential flaw could be more deleterious. In summary, it is possible to judge the significance of a flaw by detecting the direction of the flaw in the workpieces.
U.S. Pat. No. 4,480,225 discloses a dual coil bobbin eddy current inspection probe connected in a bridge circuit for an eddy current flaw detecting apparatus wherein the planes containing the dual coils are arranged in non-parallel and in predetermined acute angles to the transverse axis of the probe such that the apparatus detects flaws in a tubular workpiece in a single scan. However the apparatus does not teach determination of flaw direction in the workpiece.